Although Yersinia pestis, Y. pseudotuberculosis and Y, enterocolitica infect their host by different routes and cause diseases of variable severity, they share a common tropism for lymphoid tissues and a common capacity to resist the primary immune response of the host. They also share a highly conserved 70-kb plasmid called pYV. This plasmid encodes the adhesin YadA and eleven secreted proteins called Yops. YopM is a tyrosine phospho-protein phosphatase related to eukaryotic tyrosine phosphatases. Less is known about the ten other Yops. All the Yops are secreted by a new secretion system which is also encoded by the pYV plasmid. This new system has also been encountered in other human pathogens such as Shigella and Salmonella but, surprisingly, also in plant pathogens such as Pseudomonas solanacearum. This system seems thus to be devoted to the secretion of virulence determinants, Yop secretion is not accompanied by the removal of a N-terminal signal sequence. It requires the presence of cytoplasmic individual chaperones called ''Syc'' proteins. Yops are not freely secreted in the extracellular compartment but rather ''injected'' into eukaryotic cells when the bacterium adheres at their surface. In the case of YopH, this presumably leads to dephosphorylation of some regulatory proteins and so, prevents the antibacterial response. This represents a new mechanism in microbial pathogenesis. The chromosome of Y. enterocolitica completes the virulence panoply of Y. enterocolitica by encoding Yst, a thermostable enterotoxin related to ST1 of Escherichia coli and to guanylin, an endogenous activator of the intestinal guanylin cyclase.